Microwave circuit utilizing a semiconductor impedance element

ABSTRACT

A microwave circuit is described for use in a traveling-wave vacuum tube wherein the resistive impedance element is made of a single crystal semiconductor structure. Several microwave structures employing this element are shown.

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority Tohru Matsuoka Tokyo, Japan Apr. 22, 1968 Jan. 1 l 1972 Nippon Electric Company, Limited Tokyo, Japan Apr. 24, 1967 Japan MICROWAVE CIRCUIT UTILIZING A SEMICONDUCTOR IMPEDANCE ELEMENT 11 Claims, 5 Drawing Figs.

References Cited UNITED STATES PATENTS Washburn Hant Nalos Duncan Feinleib...

Ertel Kleinknecht Tannenbaum Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff Attorney-Sandoe, Hopgood and Calimafde US. Cl 333/31,

ABSTRACT: A microwave circuit 15 described for use In a I 333/70 333/81 g gggfgg traveling-wave vacuum tube wherein the resistive impedance 333/31 g element is made of a single crystal semiconductor structure. Several microwave structures employing this element are 81,315/3.5, 3.6, 317/235 Show.

N VE NTOR. TOHRU MA TSUOKA ATTORNEYS MICROWAVE CIRCUIT UTILIZING A SEMICONDUCTOR IMPEDANCE ELEMENT This invention relates to a slow-wave circuit for a superhigh frequency electron tube.

The structure of the conventional radiofrequency attenuator used for. a slow-wave circuit for a superhigh frequency electron tube such as a traveling-wave tube, backward wave tube or the like is such that gluey graphite called "aquadag" is sprayed over a dielectric rod normally made of glass, quartz, ceramic, or the like; or a metallic material of relatively high resistance, such as nickel, is vapor-deposited in vacuum over said dielectric rod; or aquadag is permeated into a porous ceramic material. In these conventional attenuators, the trouble is that the attenuator element essentially composed of carbon usually contains a considerable amount of gas, by which the life of the electron tube is remarkably shortened. if this kind of attenuator element consumes a large energy of high frequency, or is bombarded by an electron beam, the attenuator element will release a great amount of gas due to temperature rise thereof. This, in turn, will deteriorate the vacuum in the tube, or in some cases, will badly damage the cathode surface by the gas which has been ionized due to bombardment of the electron beam. While, the radiofrequency attenuator element which is made of a vapor deposited metallic film is effective in the application to a relatively small power device, it is liable to sputter or burn due to rapid temperature raise of the vaporized film when used in a high-power device.

One of the principal objects of this invention is therefore to provide a superhigh frequency impedance element capable of use with high power and when used in a high-power vacuum tube will release but small amounts of gaseous material.

It is a further object of the invention to provide a slow-wave structure for use at high radiofrequencies and high power.

It is still further an object to provide a slow-wave structure for obtaining a long life vacuum tube in which the structure is used.

These objects are accomplished by my invention which is described as follows in conjunction with the drawings wherein; FIGS.- 1 and 2 are perspective views respectively showing helix-type slow-wave circuits for a superhigh frequency electron tube according to the invention,

FIG. 3 is a fragmentary cross-sectional view of a cavity-cow pled type slow-wave circuit incorporating features of the invention;

FIG. 4 is a cross-sectional view along a line A-A' of FIG. 3; and

FIG. 5 is a perspective view of a Karp-type slow-wave circuit according to this invention.

Superhigh frequency energy is converted into heat energy by the resistivity of a single crystal semiconductor. Accordingly in one embodiment of my invention a superhigh frequency electron tube is provided with a radiofrequency slow-wave circuit and a radiofrequency attenuator element is formed of one or more single crystal semiconductors. The semiconductor single crystal referred to herein implies the widely known single crystals made of germanium or silicon. It is to be noted that these crystals are not limited by the impurity concentration and/or other kinds of elements contained in the crystals. In FIG. 1 a helix-type slow-wave circuit is shown comprising a helix I0 and dielectric rods 12, 14, and 16 which support said helix 10. Since the invention is not immediately related to the casing or envelope in which the helix-type slowwave circuit is enclosed or supported by and neither related to the input and output circuits of superhigh frequencies nor related to the antennae by which the input and output circuits are coupled to the slow-wave circuit, these features are not shown in the figures.

Three semiconductor single crystal rods 18, 20, and 22 are shown axially aligned with the axis of the helix l0 and distributed about the external periphery of the helix. Furthermore the rods 18, 20, and 22 are arranged in tiers as shown in FIG. 1 so that a traveling wave propagated on the slow-wave circuit will not be reflected by these attenuation elements.

FIG. 2 shows another embodiment in which a semiconductor single crystal is applied to a helix-type slow-wave circuit. In this embodiment, a part of the dielectric rod 26 supporting the helix 10 is replaced with a rod of single-crystal semiconductor. The helical slow-wave circuit comprises again a helix l0, dielectric rods 12, 14, 16, and 17 for supporting said helix 10. A rod-shaped semiconductor single crystal 24 which serves as anattenuator is disposed in alignment with and between said dielectric rods 12 and 17. The helix 10 is supported by the rod-shaped semiconductor single crystal 24 and by the dielectric rods l2, 14, 16, and 17. In order to establish the matching of the traveling wave propagating the slow-wave circuit, the end portions 23 and 25 of the rod-shaped semiconductor single crystal 24 are tapered.

In conventional microwave attenuators carbon is sprayed over a dielectric rod and the effectiveness or degree of radiofrequency attenuation depends on the state of contact between the carbon-coated dielectric rod and the helix. This contact is often unreliable causing spurious oscillations and degenerately affecting the amplifying operation of the electron tube in which the slow-wave structure is used. This disadvantage of the prior art approach is eliminated by my invention where the semiconductor single crystal 24 is fused to the helix 10 by a gold alloy diffusion technique or by brazing. intimate surface contact with the helix 10 is assured thereby and close tolerance control obtained.

FIGS. 3 and 4 illustrate a cylindrical cavity coupled type slow-wave circuit according to my invention. Referring to these figures, the slow-wave circuit comprises: conductive walls 26 forming a path of a wave; slot holes 28 through which the wave propagates; semiconductor single crystals 29 which constitute a radiofrequency attenuator of this invention; metallic cylindrical rings 30 aligned to form a central path 32 for an electron beam; and outer wall 34 of the waveguide. It is to be noted that the number of the semiconductor single crystals are not limited to four as shown in FIG. 4, but it may be one or more depending upon the desired amount of attenuation and required impedance matching.

It is to be also noted that the effect of the attenuator 29 is usually enhanced when the semiconductor single crystal is disposed close to a point where the radiofrequency electric field intensity is large, such as in the vicinity of the metallic cylindrical ring 30. This effect, however, varies depending on the size of the cavity, applied frequency, dimensions, and resistivity of the semiconductor rod. It is desirable, as in the case of FIG. 1 or 2, that the semiconductor rod can be fixed rigidly by means of gold alloy diffusion.

It is desirable to assemble the slow-wave structure by brazing in a hydrogen atmosphere. Such assembly technique tends to cause decarbonation of prior art conventional carbonsprayed attenuators, which were, therefore, attached to the slow-wave structure during an additional subsequent assembly step. My invention removes such problems and permits assembly of the slow-wave structure of FIG. 3 by brazing in hydrogen gas after the attenuator 29 is fixed to the slow-wave circuit.

FIG. 5 is a perspective view of an embodiment wherein the invention is applied to a Karp-type circuit (such as that described in the Proceedings of the I.R.E., Vol. 43, No. I, pp. 41-46, Jan., 1955) which is used for millimeter waves. This slow-wave circuit comprises: a waveguide sidewall 36; a central ridge 38; a pair of triangular plate-shaped semiconductor single crystals 40 and 42 which constitute a radiofrequency attenuator according to our invention and; a ladder-shaped circuit board or slotted wall 44, which is fixed to the waveguide sidewall 36 across the ridge 38. This slow-wave circuit is normally adopted for use in backward wave oscillation tubes. The attenuator plate-shaped semiconductor single crystals 40 and 42 are fixed to the downstream portion of the electron beam which flows in the direction of arrow 48. This attenuator serves as a terminal resistor which is impedance matched over a wide frequency range, and therefore, is shown in FIG. 5, in tapered shape.

In the Karp-type slow-wave circuit of FIG. 5, it is often the case that, in order to intensify the interaction between the electron beam and the electric field, the electron beam is so made to pass on both sides of the circuit board 44, as disclosed in the abovecited publication, to thereby sandwich the board between the upper and lower parts of the beam. This utilizes the larger electric field intensity in the vicinity of the center of the ladder-shape circuit board 44. In such case, however, a part of the electron beam occasionally flows into the attenuators 40 and 42. In the conventional attenuator formed ofa thin carbon film, the gas contained in the thin film is released when electrons of high energy flow into the attenuator and, as a result, the vacuum in the tube is deteriorated. In addition, the cathode is considerably damaged due to the ionized gas particles. l experimentally confirmed that, in the attenuator of my invention, deterioration of the vacuum is hardly noticeable.

The invention has been explained by referring to several embodiments. It is to be noted that this invention is not limited to the scope as disclosed in the specific embodiments. For example, when the semiconductor single crystal of my invention is utilized as a resistive impedance element and is employed for the purpose of suppressing spurious modes marked advantages and effects can be obtained as explained in the following paragraphs.

Since the semiconductor single crystal contains very little gas, it is hardly possible to cause deterioration of the vacuum in the electron tube in either case wherein the electron tube is operated in a high-temperature atmosphere or at a high-power output. This permits the electron tube to be operated at a high efficiency, and also ensures the electron tube to be usable for a long period oflifetime.

Since the physical property of semiconductor single crystal is quite similar to that of metal and is a good heat conductor, temperature rise in the attenuator itself can easily be restrained with a suitable heat sink.

The melting point of semiconductor single crystal is high and its vapor pressure is extremely low. Consequently, there is no possibility of causing deterioration on the conductivity of the slow-wave circuit due to sputtering. Especially in case of silicon, its stability is nearly the same as that of nickel. As a result, the circuit can be thermally processed even after assembling the attenuator, thus simplifying the manufacture of microwave electron tubes.

in the conventional attenuator, the resistive material is attached to the dielectric body in the form of a thin film which may peel off. Such peeling problem does not arise with the attenuator of my invention.

The radiofrequency attenuator constituted of a semiconductor single crystal can be rigidly fixed to a slow-wave circuit or a metallic body forming an electron tube, by means of alloy diffusion the gold alloy diffusion technique ensures intimate contact between the single crystals attenuator and the microwave circuit. This, as noted above, prevents the peelingoff of the attenuator and thus avoids spurious oscillations in the TWT while achieving effective attenuation. This is an outstanding feature of the invention, compared with the conventional attenuator which requires mechanical affixing.

Listed above are the advantages of a semiconductor single crystal when used as an attenuator for a slow-wave circuit. It is known, that semiconductor resistivity varies with temperature. However, in the attenuator used for a radiofrequency slow-wave circuit for an electron tube such as a traveling-wave tube or backward wave tube, large variations in resistivity may be tolerated. It is clarified through various experiments that, in the resistive body used for the attenuator, the variation in the attenuation as a function of changes in the resistivity is not steep at all but rather shows a low-sloped characteristic curve. Specifically, when the resistivity of the semiconductive body varies with temperature, the corresponding variation in the amount of radiofrequency attenuation against the total amount of attenuation is less then 20 percent.

Practically, when three single Si crystals having the respective resistivities of 0.001 ohm per centimeter, 0.01 ohm per centimeter and 0.1 ohm per centimeter were used for experimental purpose, the difference in these resistivities observed is not more than 20 db., against the total attenuation amount of 60 db. in the 4,000 mc. band, under the normal temperature condition.

My invention has been described in detail, and it is to be understood that the scope of my invention covers all the slowwave circuits for a superhigh frequency electron tube as described in the following claims.

I claim:

1. A microwave circuit for use in a radiofrequency vacuum tube comprising;

a microwave structure provided with a traveling electromagnetic wave path and,

a microwave impedance element bonded in electrical conducting relationship to said microwave structure and positioned generally in abutting relationship with said path,

said impedance element comprising a single crystal silicon semiconductor structure having a preselected size and being tapered to provide a minimum reflection of the wave traveling along said path towards said semiconductor structure.

2. The microwave circuit as recited in claim 1 wherein;

said semiconductor structure is brazed to said microwave structure.

3. The microwave circuit as recited in claim 1 wherein;

said semiconductor structure is gold diffusion bonded to said microwave structure.

4. The microwave circuit as recited in claim 1 wherein;

said microwave structure comprises a helix having a central axis and a plurality of semiconductor structures axially and peripherally distributed about said helix.

5. The microwave circuit as recited in claim 4 wherein;

said semiconductor structures are selectively distributed for the suppression ofspurious oscillations.

6. The microwave circuit as recited in claim 1 and further including;

a plurality of supporting elements, where one of said elements comprises said semiconductor structure.

7. The microwave circuit as recited in claim 6 wherein;

the other supporting elements comprise dielectric material,

said dielectric elements being aligned along said path, with said semiconductor structure positioned in substantial alignment with said dielectric elements for reducing reflections of electromagnetic waves traveling along the path.

8. The microwave circuit as recited in claim 1 wherein;

said microwave structure comprises a plurality of aperture coupled cavities in series relationship with one another,

and wherein said semiconductor structure is selectively positioned within one of said cavities.

9. The microwave circuit as recited in claim 1 wherein;

said semiconductor structure is rod shaped.

10. The microwave circuit as recited in claim 1 wherein;

said microwave circuit is further provided with a passageway for passing an energized beam of electrons,

and wherein said semiconductor structure is positioned adjacent to said passageway.

11. The microwave circuit as recited in claim 10 wherein a plurality of semiconductor structures are symmetrically placed about said passageway. 

1. A microwave circuit for use in a radiofrequency vacuum tube comprising; a microwave structure provided with a traveling electromagnetic wave path and, a microwave impedance element bonded in electrical conducting relationship to said microwave structure and positioned generally in abutting relationship with said path, said impedance element comprising a single crystal silicon semiconductor structure having a preselected size and being tapered to provide a minimum reflection of the wave traveling along said path towards said semiconductor structure.
 2. The microwave circuit as recited in claim 1 wherein; said semiconductor structure is brazed to said microwave structure.
 3. The microwave circuit as recited in claim 1 wherein; said semiconductor structure is gold diffusion bonded to said microwave structure.
 4. The microwave circuit as recited in claim 1 wherein; said microwave structure comprises a helix having a central axis and a plurality of semiconductor structures axially and peripherally distributed about said helix.
 5. The microwave circuit as recited in claim 4 wherein; said semiconductor structures are selectively distributed for the suppression of spurious oscillations.
 6. The microwave circuit as recited in claim 1 and further including; a plurality of supporting elements, where one of said elements comprises said semiconductor structure.
 7. The microwave circuit as recited in claim 6 wherein; the other supporting elements comprise dielectric material, said dielectric elements being aligned along said path, with said semiconductor structure positioned in substantial alignment with said dielectric elements for reducing reflections of electromagnetic waves travEling along the path.
 8. The microwave circuit as recited in claim 1 wherein; said microwave structure comprises a plurality of aperture coupled cavities in series relationship with one another, and wherein said semiconductor structure is selectively positioned within one of said cavities.
 9. The microwave circuit as recited in claim 1 wherein; said semiconductor structure is rod shaped.
 10. The microwave circuit as recited in claim 1 wherein; said microwave circuit is further provided with a passageway for passing an energized beam of electrons, and wherein said semiconductor structure is positioned adjacent to said passageway.
 11. The microwave circuit as recited in claim 10 wherein a plurality of semiconductor structures are symmetrically placed about said passageway. 